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Everything works fine if I do not add internal resistance to choke inductor in my simulations. I get 10 kHz oscillation, but when I add internal resistance to choke inductor, oscillation does not start. I am using 5 mH choke inductor with internal resistance of 50 ohms. I do not know internal resistance of this inductor as I did not prepare it yet. I will take an iron nail and wrap wire around it till I get 5mH inductance. This is my plan as I do not have such inductor available to me. When I set its internal resistance to 30 or 40 ohms, the circuit is working fine. However, when I exceed 50 ohms, it does not start working. As I do not know what resistance my home-made inductor will give me, I have to speculate every scenario. I also checked the circuit to see if it stays in forward active region and it does so with 50 ohms of collector resistor. I am very confused. Can you help me understand this and give me some advice to make this circuit function better? If I can I would totally get rid of that choke inductor, but I do not know how to.

BJT has 300 current gain value and 0.7 V V-BE turn on voltage. 30k resistor is the load.

the circuit that works

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  • \$\begingroup\$ An iron nail isn't going to be easy to use here. Iron is notorious for Eddy currents at HF. Also, an 9 V battery has somewhere between 1 and 2 or 3 Ohms of resistance. So keep that in mind in your simulations, too. If nothing else, this means you want a capacitor bypassing your 9 V battery and tied tightly and close to your circuit. I think there are youtube videos (probably lots of them) that help you fashion these kinds of circuits from readily available materials and parts. \$\endgroup\$
    – jonk
    Apr 10, 2021 at 23:06
  • \$\begingroup\$ Do you really want to make a Colpitts oscillator that works at 10kHz? That's implying a rather crazy frequency stability for audio, and big, hard-to-find parts. Have you considered a phase-shift or Wien bridge oscillator? \$\endgroup\$
    – TimWescott
    Apr 10, 2021 at 23:25
  • \$\begingroup\$ I don't know where you live, but if you can get your hands on old radios, an audio interstage coupling transformer may be about right -- or could be something that you could strip and rewind to your desired inductance. An old-style public-announcement system that uses a "70V" audio distribution system uses similar transformers, which you may find available locally. \$\endgroup\$
    – TimWescott
    Apr 10, 2021 at 23:36
  • \$\begingroup\$ @jonk: "Iron is notorious for Eddy currents at HF" 10kHz? HF? \$\endgroup\$
    – TimWescott
    Apr 10, 2021 at 23:37
  • \$\begingroup\$ @TimWescott Missed the frequency. Thanks for the catch. (I'd assumed this was another of those FM broadcast things. My mistake. Thanks.) \$\endgroup\$
    – jonk
    Apr 10, 2021 at 23:42

1 Answer 1

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C5 vs. Q1:

You have biased Q1 to operate at 1.5mA emitter current, which means that the resistance looking into the emitter of Q1 is about 17 ohms. Yet at 10kHz, C5 has an reactance of 105 ohms. The rule of thumb here is that the capacitive reactance should be 1/10 the emitter resistance at the design frequency -- so you want C5 to be about 100 times as big.

C2 and C3:

Your tank circuit (C2, C3 and L1) acts sorta-kinda like a really narrow-band transformer, with a "transformer" ratio of C2 / C3 -- and you've set C3 to be rather larger than C2. Try making these closer to equal capacitances.

L1:

You say 5mH in the text, but 0.5mH in the schematic. If it were an RF circuit, you'd definitely want to tend toward 200-500 ohms of inductive reactance, which implies 7-15mHh. 5mH is probably enough, if you actually use it.

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  • \$\begingroup\$ 5 mH is the choke - L2 - not the resonant L1. \$\endgroup\$
    – user16324
    Apr 11, 2021 at 11:16

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